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Abstract:

A method of making a water vapor transport membrane is described. The
method can include providing two sheets, each sheet comprising a support
layer with an ionomer layer thereon; applying a solvent to at least one
sheet; and contacting the ionomer layers of the two sheets to form a
composite membrane comprising a composite ionomer layer between the two
support layers. A composite membrane is also described.

Claims:

1. A method of making a water vapor transport membrane comprising:
providing two sheets, each sheet comprising a support layer with an
ionomer layer thereon; applying a solvent to at least one sheet; and
contacting the ionomer layers of the two sheets to form a composite
membrane comprising a composite ionomer layer between the two support
layers.

2. The method of claim 1 wherein the solvent is applied to the support
layer.

3. The method of claim 1 wherein the solvent is an alcohol.

4. The method of claim 1 wherein the solvent is isopropanol.

5. The method of claim 1 further comprising drying the composite
membrane.

6. The method of claim 1 wherein the composite ionomer layer has a
thickness greater than about 5 microns.

7. The method of claim 1 wherein the support layer is ePTFE.

8. The method of claim 1 wherein providing two sheets, each sheet
comprising the support layer with the ionomer layer thereon comprises:
coating an ionomer layer on a backer layer; applying the support layer to
the wet ionomer layer; drying the wet ionomer layer with the support
layer; and removing the backer layer.

9. The method of claim 1 wherein the composite membrane has a beginning
of life water vapor transfer of at least about 10,000 GPU.

10. The method of claim 1 wherein the composite membrane has a beginning
of life water vapor transfer of at least about 14,000 GPU.

11. The method of claim 1 wherein the composite membrane has a beginning
of life water vapor transfer of at least about 16,000 GPU.

12. The method of claim 1 wherein the two ionomer layers are made of
different ionomer.

13. The method of claim 1 wherein the two support layers are made of
different materials.

14. A method of making a water vapor transport membrane comprising:
coating an ionomer layer on a backer layer; applying a support layer to
the wet ionomer layer; drying the wet ionomer layer; removing the backer
layer to form a sheet comprising a support layer with an ionomer layer
thereon; applying a solvent to the support layer of at least one sheet;
and contacting the ionomer layers of two sheets to form a composite
membrane comprising a composite ionomer layer between the first and
second support layers.

15. The method of claim 14 wherein the composite membrane has a beginning
of life water vapor transfer of at least about 10,000 GPU.

16. The method of claim 14 wherein the two ionomer layers are made of
different ionomers.

17. The method of claim 14 wherein the two support layers are made of
different materials.

18. A composite membrane comprising: a composite ionomer layer between a
pair of support layers, the composite ionomer layer comprising at least
two ionomer sublayers.

19. The composite membrane of claim 18 wherein the two ionomer sublayers
are made of different ionomers.

20. The composite membrane of claim 18 wherein the pair of support layers
are made of different materials.

21. The composite membrane of claim 18 wherein the composite membrane has
a beginning of life water vapor transfer of at least about 10,000 GPU.

22. The composite membrane of claim 18 wherein the composite membrane has
a beginning of life water vapor transfer of at least about 14,000 GPU.

23. The composite membrane of claim 18 wherein the composite membrane has
a beginning of life water vapor transfer of at least about 16,000 GPU.

Description:

BACKGROUND OF THE INVENTION

[0001] This invention relates generally to a fuel cell humidifier unit,
and more particularly to the construction of a water-permeable membrane
within the humidifier and a method for making the same.

[0002] In many fuel cell systems, hydrogen or a hydrogen-rich gas is
supplied through a flowfield to the anode side of a fuel cell while
oxygen (such as in the form of atmospheric oxygen) is supplied through a
separate flowfield to the cathode side of the fuel cell. An appropriate
catalyst (for example, platinum) is typically disposed as a layer on
porous diffusion media that is typically made from a carbon fabric or
paper such that the combination exhibits resiliency, electrical
conductivity, and gas permeability. The catalyzed diffusion media is used
to facilitate hydrogen oxidation at the anode side and oxygen reduction
at the cathode side. An electric current produced by the dissociation of
the hydrogen at the anode is passed from the catalyzed portion of the
diffusion media and through a separate circuit such that it can be the
source of useful work, while the ionized hydrogen passes through another
medium situated between the diffusion media of the anode and cathode.
Upon such passage, the ionized hydrogen combines with ionized oxygen at
the cathode to form high temperature water vapor as a reaction byproduct.
In one form of fuel cell, called the proton exchange membrane or polymer
electrolyte membrane (in either event, PEM) fuel cell, the medium for
ionized hydrogen passage is an electrolyte in the form of a
perfluorinated sulfonic acid (PFSA) ionomer membrane (such as
Nafion®). This layered structure of the membrane surrounded on
opposing sides by the catalyzed diffusion media is commonly referred to
as a membrane electrode assembly (MEA), and forms a single fuel cell.
Many such single cells can be combined to form a fuel cell stack,
increasing the power output thereof.

[0003] Fuel cells, particularly PEM fuel cells, require balanced water
levels to ensure proper operation. For example, it is important to avoid
having too much water in the fuel cell, which can result in the flooding
or related blockage of the reactant flowfield channels. On the other
hand, too little hydration limits the electrical conductivity of the
membrane and can lead to premature cell failure. Increasing the
difficulty in maintaining a balance in water level is that there are
numerous conflicting reactions taking place in a fuel cell that are
simultaneously increasing and decreasing local and global hydration
levels.

[0004] One method of ensuring adequate levels of hydration throughout the
fuel cell includes humidifying one or both of the reactants before they
enter the fuel cell. For example, residual water present at the cathode
exhaust can be used with an appropriate humidification device to reduce
the likelihood of dehydration of the anode, the PFSA ionomer membrane, or
the cathode inlet. One such humidification device is a water vapor
transfer (WVT) unit, which may also be referred to as a membrane
humidifier, fuel cell humidifier, or related assembly. The WVT unit
extracts the moisture from a humid fuel cell exhaust flowpath and places
it into the feed path of a reactant low in humidity. Wet-side and
dry-side reactant flowpaths (for example, a cathode exhaust and a cathode
inlet) are in moisture-exchange communication with one another in the WVT
unit through one or more separators (also known as separator plates). In
a particular manufacturing approach, the separator is formed continuously
as a roll with a pair of planar porous layers and a support spaced apart
by elongated strings placed between them. From this continuous roll, the
WVT unit can be cut into sizes and shapes needed for a particular fuel
cell application. Examples of WVT units may be found in U.S. Pat. Nos.
7,749,661 and 7,875,396, as well as US Published Patent Application
2009/0092863, all of which are assigned to the assignee of the present
invention and the entire contents of which are herein incorporated fully
by reference.

[0005] The exchange of humidity is generally accomplished in the WVT unit
by using an ionomer membrane disposed between adjacent high humidity and
low humidity fluid flowpaths formed in the separators. The generally
planar membrane (which may structurally resemble that of the PFSA
membrane discussed above) allows water vapor to pass from the higher
humidity fluid on one side to the lower humidity fluid on the other while
inhibiting the direct mixing of the two fluids, for example, the cathode
inlet and cathode exhaust that are being conveyed through the flowpaths.
In one form of construction, the ionomer membrane is attached to an
adjacent support layer (which may be a thin layer of expanded
poly(tetrafluoroethylene) (ePTFE) or related material to increase the
robustness and handleability of the membrane and support, as well as to
avoid migration of the ionomer into a porous support. Despite such
layering, the combination remains fragile where, in one typical form, the
ionomer membrane may be between 3 and 10 microns thick, while the support
(made, for example, from expanded poly(tetrafluoroethylene) (ePTFE)) may
be about 10 to 30 microns thick and collapse down to about 5 to about 20
um thick when in contact with the dispersion.

[0006] Existing processes associated with WVT unit fabrication may expose
the ionomer membrane and support to excessive handling and related harsh
conditions that could jeopardize membrane integrity through failure modes
such as crossover. Likewise, existing processes that end up scrapping
significant portions of ionomer membrane result in waste and related cost
increases. As stated above, the separator is produced in roll form; in
such form, the ionomer membrane is on a supporting layer of backer
material which must be removed as part of the separator plate
manufacturing process. These features significantly contribute to the
overall cost of incorporating ionomer membranes into the WVT unit. As
such, approaches that would reduce both the handling and cost issues
associated with such membrane material are preferred.

SUMMARY OF THE INVENTION

[0007] One aspect of the invention is a method of making a water vapor
transport membrane. In one embodiment, the method includes providing two
sheets, each sheet comprising a support layer with an ionomer layer
thereon; applying a solvent to at least one sheet; and contacting the
ionomer layers of the two sheets to form a composite membrane comprising
a composite ionomer layer between the two support layers.

[0008] Another aspect of the invention is a composite membrane. In one
embodiment, the composite membrane includes a composite ionomer layer
between a pair of support layers, the composite ionomer layer comprising
at least two ionomer sublayers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The following detailed description of specific embodiments can be
best understood when read in conjunction with the following drawings,
where like structure is indicated with like reference numerals and in
which:

[0010] FIG. 1 shows a simplified version of a WVT unit and the
humidity-exchange flowpaths therethrough.

[0011] FIGS. 2A-C show one embodiment of a process for preparing a water
vapor transport membrane according to an aspect of the present invention.

[0012]FIG. 3 is a graph showing the WVT for samples made according to
different embodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] FIG. 1 illustrates a WVT unit 1 with multiple layers of wet and dry
separators for the exchange of moisture. In one form, the wet flow 2 may
be that coming from a fuel cell cathode exhaust, while the dry flow 4 may
be the inlet to the fuel cell cathode, although it will be appreciated
that other flowpaths are also within the scope of the present invention.

[0014] The method allows the production of leak-free WVT membranes having
thick ionomer layers, which provides greater durability and longer life
for the membrane. The process is simple and scalable. It allows WVT core
manufacturability with existing assembly methods. It reduces the
potential for defects to affect performance because the defects in each
sheet would have to line up in order to provide a leak path. It will also
allow the preparation of a WVT membrane with different ionomer layers
and/or different support layers on each side.

[0015] By "leak-free," we mean less than 2% air crossover throughout the
life of the WVT unit, less than 150 GPU, measured with a set pressure
difference between the two sides, typically 3 psi.

[0016] FIG. 2 shows one embodiment of a method of making a WVT membrane
according to the present invention. As shown in FIG. 2A, the ionomer
membrane layer 45 is coated onto a backer layer 50, and support layer 55
is wet laminated to ionomer membrane layer 45. The backer layer 50 is
then removed. As shown in FIG. 2B, solvent is applied to the support
layer 55 side of one or both of the ionomer membrane structures. The
solvent can be applied using any suitable process, such as spraying. The
solvent permeates the support layer 55 and the ionomer membrane layer 45
and makes the ionomer tacky. Alternatively, the solvent can be applied to
the ionomer side of the structure. If the solvent is applied to the
ionomer side, it could also include ionomer, if desired. Any solvent that
makes the ionomer tacky can be used. Suitable solvents include, but are
not limited to, alcohols, such as isopropanol.

[0017] As shown in FIG. 2C, the ionomer membrane layer 45 sides of two
ionomer membrane structures are placed in contact to make a composite
membrane 80 including the composite ionomer layer 85 sandwiched between
the two support layers 55. When the structure is dry, the two ionomer
membrane layers are glued together. Simple contact is sufficient to join
the two ionomer membrane layers together. Neither heat nor compression
are required to form the composite membrane 80, although either or both
can be used, if desired.

[0018] Suitable ionomers include, but are not limited to PFSA ionomer,
such as for example Nafion®, or Aquivion®, or
perfluorocyclobutane (PFCB) ionomer. One suitable PFSA ionomer is
Aquivion® D70-20BS, which is a short side chain PFSA-based ionomer
with a 700 EW. Suitable PFCB ionomers are described in U.S. application
Ser. Nos. 12/549,881, 12/549,885, and 12/549,904, each of which is
incorporated herein by reference.

[0019] The final composite ionomer layer is typically greater than about 5
microns in thickness, or greater than about 6 microns in thickness, or
greater than about 7 microns in thickness, or greater than about 8
microns in thickness, or greater than about 9 microns in thickness, or
greater than about 10 microns in thickness, or greater than about 11
microns in thickness, or greater than about 12 microns in thickness, or
greater than about 13 microns in thickness, or greater than about 14
microns in thickness, or greater than about 15 microns in thickness, or
about 5 to about 15 microns, or about 6 to about 15 microns, or about 7
to about 15 microns, or about 8 to about 15 microns, or about 9 to about
15 microns, or about 5 to about 10 microns, or about 6 to about 10
microns, or about 7 to about 10 microns, or about 8 to about 10 microns,
or about 9 to about 10 microns. The tradeoff is performance, cost, and
durability.

[0020] The composite membrane desirably has a beginning of life water
vapor transfer of at least about 8,000 GPU, or at least about 10,000 GPU,
or at least about 12,000 GPU, or at least about 14,000 GPU, or at least
about 15,000 GPU, or at least about 16,000 GPU. For a compact fuel cell
humidifier application, the membrane will generally have a permeance of
greater than about 8,000 gas permeation units (GPU) (GPU is a partial
pressure normalized flux where 1 GPU=10-6 cm3 (STP)/(cm2
sec cm Hg)), and typically in the range of about 10,000-12,000 GPU for a
25 μm homogeneous Nafion®. Beginning of life indicates the
performance within the first twenty hours following any break-in period.

Example

[0021] A backer material of fluorinated ethylene-propylene (FEP)-coated
polyimide film backer material (e.g., Kapton® 120FN616, 1 mil
available from DuPont), was coated with PFSA ionomer. An ePTFE support
layer was laid down on the wet ionomer. After drying, the backer material
was removed. The ionomer layer was 3-10 microns thick. The ePFTE side of
one sheet was sprayed with isopropanol, and the ionomer side of a second
sheet was hand pressed onto it at room temperature. The pressure on a
soft roller was contact pressure sufficient to remove any trapped air
bubbles between layers. The final composite had a first ePTFE layer about
4.1 microns thick, a 5.4 micron thick composite ionomer layer, and a
second ePTFE layer 4.8 microns thick.

[0022] A second composite membrane was made by coating a backer material
of fluorinated ethylene-propylene (FEP)-coated polyimide film backer
material (e.g., Kapton® 120FN616, 1 mil available from DuPont), with
PFCB ionomer, and applying ePTFE to the wet ionomer. The ePFTE side of a
sheet was sprayed with isopropanol, and the ionomer side of two sheets
were hand pressed together at room temperature.

[0023] The water vapor transfer of the single sheet and composite membrane
made with Aquivion® D70-20BS (Samples 1 and 2) and with PFCB (Samples
3 and 4) were measured. Samples 1 and 3 were single sided structures of
ionomer/ePTFE, and samples 2 and 4 were double sided composite structures
of ePTFE/ionomer composite/ePTFE. The composite membrane made with
Aquivion® D70-20BS (Sample 2) exceeded a beginning of life water
vapor transfer of 16,000 GPU (GPU is a partial pressure normalized flux
where 1 GPU=10-6 cm3 (STP)/(cm2 sec cm Hg)). The composite
membrane made with PFCB (Sample 4) had a beginning of life water vapor
transfer of almost 10,000 GPU.

[0024] The beginning of life water vapor transfer was measured using a 50
cm2 membrane area, and straight flowfields with a similar geometry
to that shown in U.S. Pat. No. 7,875,396, counter flow, with a dry side
flow of 11.5 slpm, 80 C, 183 kPaa, and wet side flow of 10 slpm, 80 C,
160 kPaa. It is noted that terms like "preferably," "commonly," and
"typically" are not utilized herein to limit the scope of the claimed
invention or to imply that certain features are critical, essential, or
even important to the structure or function of the claimed invention.
Rather, these terms are merely intended to highlight alternative or
additional features that may or may not be utilized in a particular
embodiment of the present invention.

[0025] For the purposes of describing and defining the present invention
it is noted that the term "device" is utilized herein to represent a
combination of components and individual components, regardless of
whether the components are combined with other components. For example, a
"device" according to the present invention may comprise an
electrochemical conversion assembly or fuel cell, a vehicle incorporating
an electrochemical conversion assembly according to the present
invention, etc.

[0026] For the purposes of describing and defining the present invention
it is noted that the term "substantially" is utilized herein to represent
the inherent degree of uncertainty that may be attributed to any
quantitative comparison, value, measurement, or other representation. The
term "substantially" is also utilized herein to represent the degree by
which a quantitative representation may vary from a stated reference
without resulting in a change in the basic function of the subject matter
at issue.

[0027] Having described the invention in detail and by reference to
specific embodiments thereof, it will be apparent that modifications and
variations are possible without departing from the scope of the invention
defined in the appended claims. More specifically, although some aspects
of the present invention are identified herein as preferred or
particularly advantageous, it is contemplated that the present invention
is not necessarily limited to these preferred aspects of the invention.